Adjustments in the rate of renal solute and water excretion constitute the major defense of man's extracellular fluid volume against potentially disruptive effects of variations in the intake and extrarenal loss of water and solutes. The research program described herein is directed toward improving our awareness of some of the mechanisms operating at the level of the individual nephrons and renal capillaries which contribute to this homeostatic ability. As such, micropuncture techniques augmented with a computer assisted television microscopy system for determining erythrocyte velocities, microvessel diameters and capillary hematocrit will be used to examine the effects of experimentally induced alterations in renal solute and water transport in the inner medullary region of the intact rodent kidney. Specifically, in the Munich-Wistar rat, the relationship between medullary blood flow, as inferred from direct measurements on individual vasa recta, and plasma levels of ADH and PGE1 will be determined. In addition, a comparison of the renal macro-and microcirculation will be made between the Munich-Wistar rat an Psammomys obesus.